Two explosions have rocked the southern Philippine port city of Zamboanga, killing five people and injuring at least 144, police said.

The blasts, which police said were caused by bombs, occurred within minutes of each other, in what appeared to be a co-ordinated attack.

The Philippines, like elsewhere in the region, is currently on a heightened security alert following the weekend bomb attack in Bali which killed almost 200 people.

The restive southern part of the country is no stranger to explosions, which are often blamed on Muslim extremist groups or extortion rackets.

A spokesman for the military told the news agency AFP that the prime suspect for Thursday's attack was a branch of the Muslim rebel Abu Sayyaf group.

But the list of possible suspects also included regional group Jemaah Islamiah, which has been named by some governments as being involved in Saturday's bomb attacks on the Indonesian island of Bali.

Several bombs

The first blast was at the three-storey Shop-O-Rama department store, according to police.

The police said they then found a second bomb in the store, and that as they were defusing it, a third bomb went off in Shopper's Central, a grocer's store less than a block away.

There was a fatal bombing in the south only two weeks ago

Witnesses at the scene of the first attack said a suspect was seen leaving a package inside the Shop-O-Rama store prior to the explosion, police said.

Shattered glass and debris were scattered everywhere. One man was reported to have been thrown through a plate glass window.

The injured were taken to a number of local hospitals for treatment.

The latest attacks come just hours after the new head of the army southern command, Lieutenant General Narciso Abaya, assumed office, vowing to crush the rebels.

Military spokesman Lieutenant Colonel Danilo Servando told AFP: "We are looking at the (Abu Sayyaf) group of Khadaffy Janjalani as primarily responsible for the incident."

But he stressed he was not ruling out other suspects.

Possible suspects

Two weeks ago a home-made bomb near a karaoke bar in Zamboanga killed at least three people, including a US soldier.

Police blamed that explosion on the Abu Sayyaf, which the US has linked to the al-Qaeda network, but which most analysts agree is simply engaged in kidnap for ransom.

The BBC's correspondent in Manila, John Mclean, says businesses in the southern Philippines are also prey to simple extortion gangs, and at this stage, the perpetrators of Thursday's bombs could have been anyone.

Zamboanga is a predominantly Roman Catholic city in the middle of the Philippines' Muslim heartland.

US forces have been helping to train Filipino soldiers to fight the Abu Sayyaf guerrillas, who are active in the south.